1. Field of the Invention
This invention relates to the fluidized catalytic cracking (FCC) conversion of heavy hydrocarbons into lighter hydrocarbons with a fluidized stream of catalyst particles and regeneration of the catalyst particles to remove coke which acts to deactivate the catalyst. More specifically, this invention relates to the apparatus for performing the FCC process.
2. Description of the Prior Art
Catalytic cracking is accomplished by contacting hydrocarbons in a reaction zone with a catalyst composed of finely divided particulate material. The reaction in catalytic cracking, as opposed to hydrocracking, is carried out in the absence of added hydrogen or the consumption of hydrogen. As the cracking reaction proceeds, substantial amounts of coke are deposited on the catalyst. A high temperature regeneration within a regeneration zone operation burns coke from the catalyst. Coke-containing catalyst, referred to herein as spent catalyst, is continually removed from the reaction zone and replaced by essentially coke-free catalyst from the regeneration zone. Fluidization of the catalyst particles by various gaseous streams allows the transport of catalyst between the reaction zone and regeneration zone.
Methods for cracking hydrocarbons in a fluidized stream of catalyst, transporting catalyst between reaction and regeneration zones, and combusting coke in the regenerator are well known by those skilled in the art of FCC processes. To this end, the art is replete with vessel configurations for contacting catalyst particles with feed and regeneration gas, respectively. These different configurations of FCC units include a stacked reactor/regenerator system where the FCC reactor is located directly above an FCC regenerator and a side-by-side FCC configuration where a reactor vessel is located to the side and above an FCC regenerator. Another form of reactor/regenerator configuration has a two-stage regeneration vessel with a reactor vessel again located to the side of the regenerator. In the two-stage regeneration vessel, there is usually an upper and lower regeneration chamber either or both of which may contain a dense fluidized bed of catalyst. In all of these configurations, regenerated catalyst flows from a regeneration vessel through a regenerator standpipe into a riser where it contacts an FCC charge stock. Expanding gases from the charge stock and fluidizing medium convey the catalyst up the riser and into a reactor vessel. Cyclone separators in the reactor divide the catalyst from reacted feed vapors which pass into an upper recovery line while the catalyst collects in the bottom of the reactor. A stripping vessel, located below the reactor vessel, receives spent catalyst from the reaction zone. Steam rises from the bottom of the stripper, countercurrent to the downward flow of catalyst, and removes sorbed hydrocarbons from the catalyst. Spent catalyst continues its downward movement from the stripper vessel through a reactor standpipe and into a dense fluidized catalyst bed contained within the regeneration vessel. Coke on the spent catalyst reacts with oxygen in air stream that ascends through the regeneration vessel and ultimately becomes regeneration gas. Again, cyclone separators at the top of the regenerator return catalyst particles to the dense bed and deliver a relatively catalyst-free regeneration gas to an overhead gas conduit.
Changes in regeneration technique, types of available feedstock, and higher throughput requirements have affected the way in which FCC units are operated. These operational changes have greatly diminished the utility and viability of many existing FCC arrangements. A particularly useful addition to the regeneration technique are means to remove heat from the regenerator. The major impetus for adopting heat removal techniques in the regenerator is the need to improve conversion of a wide variety of feedstocks.
Optimization of feedstock conversion ordinarily requires essentially complete removal of coke from the catalyst. This essentially complete removal of coke from catalyst is often referred to as complete regeneration. Complete regeneration produces a catalyst having less than 0.1 and preferably less than 0.05 weight percent coke. The application of the FCC process to crack heavy feedstocks produces greater amounts of coke. With the increased coke producing tendencies of these heavy or residual feeds, a complete regeneration of catalyst becomes more difficult due to the excessive heat evolution associated with additional coke combustion.
The increase in coke on spent catalyst results in a larger amount of coke being burnt in the regenerator per pound of catalyst circulated. Heat is removed from the regenerator in conventional FCC units in the flue gas and principally in the hot regenerated catalyst stream. An increase in the level of coke on spent catalyst will increase the temperature difference between the reactor and the regenerator, and the regenerated catalyst temperature overall. A reduction in the amount of catalyst circulated is, therefore, necessary in order to maintain the same reactor temperature. However, this lower catalyst circulation rate required by the higher temperature difference between the reactor and the regenerator will lower hydrocarbon conversion, making it necessary to operate with a higher reactor temperature in order to maintain conversion at the desired level. This will cause a change in yield structure which may or may not be desirable, depending on what products are required from the process. Also, there are limitations to the temperatures that can be tolerated by FCC catalyst without there being a substantial detrimental effect on catalyst activity. Generally, with commonly available modern FCC catalyst, temperatures of regenerated catalyst are usually maintained below 760.degree. C. (1400.degree. F.), since loss of activity would be very severe at about 760.degree.-790.degree. C. (1400.degree. -1450.degree. F.). If a relatively common reduced crude such as that derived from Light Arabian crude oil were charged to a conventional FCC unit, and operated at a temperature required for high conversion to lighter products, i.e., similar to that for a gas oil charge, the regenerator temperature would operate in the range of 870.degree.-980.degree. C. (1600.degree.-1800.degree. F.). This temperature would be too high a temperature for the catalyst, require very expensive materials of construction, and give an extremely low catalyst circulation rate. It is, therefore, accepted that when materials are processed that would give excessive regenerator temperatures, a means must be provided for removing heat from the regenerator, which enables a lower regenerator temperature, and a lower temperature difference between the reactor and the regenerator to be obtained.
A common prior art means for removing heat from a regenerator provides coolant filled coils within the regenerator which are in contact with the catalyst. For example, Medlin et al. U.S. Pat. No. 2,819,951, McKinney U.S. Pat. No. 3,990,992, and Vickers U.S. Pat. No. 4,219,442 disclose fluid catalytic cracking processes using dual zone regenerators with cooling coils positioned in the second zone. The prior art is also replete with disclosures of FCC processes which utilize dense or dilute phase regenerated fluid catalyst heat removal zones or heat exchangers, that are external to the regenerator vessel, to cool hot regenerated catalyst for return to the regenerator. Examples of such disclosures are as set forth in Harper U.S. Pat. No. 2,970,117; Owens U.S. Pat. No. 2,873,175; McKinney U.S. Pat. No. 2,862,798; Watson et al. U.S. Pat. No. 2,596,748; Jahnig et al. U.S. Pat. No. 2,515,156; Berger U.S. Pat. No. 2,492,948; Watson U.S. Pat. No. 2,506,123; and Lomas et al. U.S. Pat. No. 4,434,245. Another U.S. Pat. No. 4,439,533 issued to Lomas et al. shows an external heat removal zone in which catalyst is circulated between the heat removal zone and the regeneration vessel across a single passage that communicates the two zones.
External heat removal zones comprising catalyst coolers having a remote location from the regenerator vessel are widely accepted. This type of cooler has been found to have a high heat withdrawal capacity, operate reliably and provide few additional constraints on the start-up, shut-down and operation of the FCC system. Two types of remote catalyst coolers have found widespread use. One is a flow-through cooler where catalyst is taken from the regenerator vessel, added to one end of the catalyst cooler, passed through the cooler, recovered from an opposite end of the cooler and returned to the regeneration vessel. The other type of catalyst cooler is a backmix cooler in which a large diameter opening at an upper end of the cooler communicates with a catalyst bed within the regeneration zone. Air added at the bottom of the cooler fludizes and backmixes the catalyst so that hot catalyst is recirculated down the length of the cooler and circulated across the opening between the catalyst cooler and the regeneration vessel. Incorporating either type of catalyst cooler into the regenerator requires clearance for the space occupied by the cooler and adequate space for cooler maintenance which normally requires of the heat exchange tubes.
Thus, adding a remote catalyst cooler to a regenerator vessel requires a clear area around the vessel where the cooler may be added and maintained. Finding the space for locating either type of cooler poses additional design constraints. These constraints apply whether the regenerator vessel is newly designed or the regenerator vessel is one that has been in service (hereinafter referred to as an existing regenerator.)
Transfer of catalyst through the flow-through type cooler normally requires more space for catalyst transfer lines around the cooler. The backmix type cooler has the advantage of not requiring transfer lines but it does require a large opening in the regeneration vessel for communicating catalyst back and forth between the cooler and the regeneration vessel. Providing the opening for a backmix catalyst cooler requires that a large opening be cut into the shell of the regenerator vessel. Depending on the size of the catalyst cooler, the required opening in the regenerator vessel will usually exceed six feet and often be greater than eight feet in diameter. Providing a large opening of this type in an existing regenerator, especially after it is operated for a substantial period of time, should be avoided where possible in order to preserve the structural integrity of the vessel. In addition, a large amount of welding will be needed to install a nozzle about the opening to which the catalyst cooler can be attached and to reinforce the regenerator vessel for the increased stresses associated with the new opening. This reinforcement will usually consist of large metal pads that surround the new nozzle and are welded to the vessel shell.
In many cases, clearance problems also pose significant difficulties. Usually, the regenerator vessel is the lowermost vessel in the reactor regenerator combination. The remote catalyst cooler normally withdraws catalyst off of the dense bed in the regenerator. Therefore, little ground clearance is usually available for the cooler, since the regenerator vessel has a relatively low elevation and the regenerator dense bed is located at a relatively low location in the regenerator vessel. In addition to ground clearance problems, the resulting low elevation of the remote cooler also limits the amount of space that will be readily available for the cooler. Much of the structure for supporting the FCC unit and a large amount of piping associated with the FCC unit or other process units are located at relatively low elevations. Therefore all of this equipment and structure can limit the amount of space available for the cooler.
As a result, space constraints and mechanical considerations associated with the regenerator vessel make it difficult to install catalyst coolers on regenerator vessels. In particular, it would be highly desirable to add remote catalyst coolers to existing regenerator units in order to upgrade their operation for newer regeneration techniques, however, mechanical and structural limitations are the most severe on the existing vessels.
It is an object of this invention to facilitate the installation of remote catalyst coolers on regeneration vessels.
It is a further object of this invention to find a location for a remote catalyst cooler on a regeneration vessel that will readily provide clearance for the cooler and to provide an unrestricted catalyst flow to the catalyst cooler.
It is a further object of this invention to minimize the necessary structural changes to an existing regeneration vessel for the incorporation of the remote catalyst cooler.
It is a yet further object of this invention to provide a method for retrofitting a remote catalyst cooler on an existing regeneration vessel of an FCC unit.